Optical transmission systems typically include a transmitter (Tx) and a receiver (Rx) interconnected by an optical fiber link, which may be composed of one of more fiber spans.
The optical transmissions can result in nonlinear optical impairments affecting the transmitted optical signal essentially as noise. Often, identifying the magnitude of some of the nonlinear optical impairments can be beneficial for obtaining a better understanding on the received net system margin. These impairments can include cross-phase modulation (XPM), self-phase modulation (SPM), cross-polarization modulation (XPolM), and four wave mixing (FWM). In particular, the assessment of these impairments is needed to estimate various link budget parameters, including the required optical signal-to-noise ratio to achieve a specified bit-error-ratio at the Rx, and the signal power at the input to each optical fiber span that maximizes the received net system margin.
For optical communication systems that use polarization multiplexing, two optical signals are polarized orthogonally with respect to each other before transmitted in an optical channel at a certain wavelength. This doubles the bandwidth of the channel. To realize a high-speed transmission over 100 Gbps, polarization multiplexing and dense-wavelength-division multiplexing (DWDM) are often used.
However, alternating bit sequences of the polarization multiplexing signal causes the resulting polarization to change over time. This depolarization effect results in cross-polarization modulation (XPolM), which is essentially noise, as noted above. The XPolM impairment is induced by inter-channel fiber nonlinearity particularly for DWDM optical communication systems. Due to the XPolM, the dual-polarized (DP) signals, which originally were polarized orthogonally, are no longer so, and cannot be accurately separated at the receiver.
The XPolM causes rotations of the state of polarization around an axis which itself exhibits random walk on a sphere. The XPolM becomes prominent in systems with DWDM waveforms transmitted over long, dispersion managed (DM) links. The XPolM results in significant performance degradation in bit error rate or in quality factor for dual-polarized coherent optical systems.
Accordingly, there is a need for a method for decoding symbols transmitted over an optical communication channel having a XPolM impairment.